1. Field of the Invention
The invention generally relates to variable gain amplifiers and applications of the same. In an embodiment, the variable gain amplifier is used in a set-top control box for the delivery of cable television service to a customer. In another embodiment, the variable gain amplifier is used in a cable modem.
2. Related Art
In modern sub-micron semiconductor processes, power supply voltages continue to be reduced. In an example used herein, the power supply voltage is 3.3V. This is less than the more common value of 5V used by most bipolar processes. More recent complementary metal-oxide-semiconductor (CMOS) processes operate at 1.2V. Because of this reduced power supply voltage, it is no longer possible to xe2x80x9cstackxe2x80x9d transistors on top of one another to improve bandwidth and linearity. There is simply not enough voltage dynamic range. In fact, even a simple differential amplifier may not have enough dynamic range to operate properly when the signals at the output have large amplitudes.
The need for very good linearity is even more striking when the input to the chip is single-ended, rather than differential. This requires even more dynamic range at the output of the amplifier. Being able to apply single-ended signals to the chip avoids the cost of an external balun transformer. What is needed is a system and method for improving the dynamic range of an amplifier while providing protection to the on-chip components.
This invention uses external surface mount inductors or surface mount ferrite beads to connect the output nodes of multiple differential amplifiers to VDD (the positive power supply). The external inductors or ferrites provide a short-circuit at DC and a high impedance over a range of operating frequencies (e.g., 50-860 MHz). This allows for much greater dynamic range on the internal differential pairs.
A further feature of this invention is a special on-chip biasing arrangement at the output that prevents damage from occurring to the chip, should one of the above-mentioned inductors or ferrites not be installed, or become an open-circuit.